This invention relates to a method and apparatus for the high efficiency filtering of particulate matter from a conveying fluid, such as air. The concept disclosed also has application in the filtering of gasses, as well as liquids. Filtration takes place by building a mat or layer of particulate matter upon a filter support and then using the mat as a filter medium.
The particular disclosure of this application is of a filter for removing various types of dust and fibers such as are commonly found in textile processing environments from a conveying air stream.
It is well known in the filtration art that a bare filter, such as a screen or filter fabric is a poor filter, especially of fine dust and fiber particles. This problem is an inherent one in the prior art use of filter screens and fabrics since the reduction of the size of the filter opening to trap small particles results in rapid clogging of the filter, greater energy consumption to move air through the relatively small openings, and frequent removal of the accumulated particulates from the filter surface. In contrast, it is well known that, given optimum filter opening size, efficiency improves dramatically after the filter media has collected a sufficient amount of dust on the screen, since dust is its own best filter. As is apparent, however, the dust cannot be collected forever. Upon reaching an optimum mat thickness, filtration efficiency ceases to improve. Moreover, as the increasing thickness of the mat progressively impedes the flow of air through the filter, filtration capacity at any given energy consumption level quickly decreases. For this reason, the mat must be removed periodically from at least a portion of the fiber mat in order to facilitate passage of air through the filter. Whenever the mat is removed from a portion of the filter, the pressure drop through this area decreases relative to the portion of the filter still containing the mat. As a result, most of the air entering the filter will move toward the area of least resistance and will pass through the clean filter. The result is poor filtration efficiency until the mat of dust and fibers again begins to accumulate on the filter surface. The repeated necessity of removing a portion of the mat from the filter material results in relatively short periods of time during which filtering efficiency is very high, interrupted by relatively longer periods of time where filtration efficiency is very poor. Clearly, the average filtration efficiency over an extended period of time is much lower than the efficiency of the filter when the mat is at its optimum thickness.
A first step toward the solution of this problem is disclosed in a recent patent to the present assignee (U.S. Pat. No. 4,226,715, assigned to the Terrell Machine Company). The invention disclosed in the above-identified patent was invented by the present applicant, jointly with Robert E. Terrell. As disclosed in the U.S. Pat. No. 4,226,715 patent, the area within the filter enclosure is divided into two physically separated chambers straddling the upstream and downstream sides of the filtering drum. The air to be filtered first enters the first chamber, passes through the filter medium and is removed on the downstream side of the chamber by fan into the upstream side of the physically separate second chamber. The air then passes through the fiber mat on the filter drum in the second chamber where it is again filtered, this time at a very high efficiency, and is then exhausted. The first and second chambers are physically separated by means of a rubber seal on the downstream side of the filter drum. The first and second chambers are physically separated on the upstream side of the filter drum by an imperforate wall and a polished, steel roller which engages the fiber mat and, under optimum conditions, prevents the intercommunication of air between the first and second chambers. However, it appears that a certain amount of dust and fiber was escaping from the filter which should have been trapped in the mat. It was first thought that the roller was not engaging the fiber mat tightly enough, permitting minute currents of air having relatively high velocities to pass through the mat at the point where engaged by the roller and removing and carrying through to the downstream side of the second chamber particles of dust and fiber. However, when the roller was adjusted to more tightly engage the mat as it passed beneath, filtration efficiency decreased as a result of the steel roller crushing dust and fiber particles adjacent the filter drum, allowing them to escape into the downstream side of the second chamber.
It was then thought that the problem could be solved by loosening the roller. However, it was discovered that when air is permitted to leak between the roller and fiber mat, air currents erode and eventually remove sections from the fiber mat or, at the very least, result in uneven mat thickness. This again resulted in decreased filtering efficiency.
By operating the filter fan while the apparatus was partially disassembled, i.e., with the roller on the upstream side of the filter separating the first and second chambers removed, filtering efficiency was greatly improved. Experimentation with the physical barrier on the upstream side of the first and second chambers removed revealed that uniformity of mat thickness was enhanced and the problem of crushed dust and fiber particles escaping through the filter drum into the downstream side of the second chamber was completely eliminated.